With the increasing popularity of the Internet, there has been a corresponding increase in the demand for high rate digital transmission over the local subscriber loops of telephone companies. A loop is a twisted-pair copper telephone line coupling a user or subscriber telephone to a central office (CO).
Traditionally, data communication equipment uses the voice band of the subscriber loop. Such equipment includes voice band modems, which operate at up to 56 kbps using compression techniques. On the other hand, Integrated Services Digital Network (ISDN) systems have boosted data rates over existing copper phone lines to 128 kbps. However, traditional voice band equipment is limited by the maximum data rate of the existing switching networks and PCM (Pulse Code Modulation) data highways.
Utilization of the frequency bandwidth of the loop outside the voiceband has enabled other high-speed systems to evolve. However, because loops can differ in distance, diameter, age and transmission characteristics depending on the network, they pose some limitations and challenges for designers of these high-speed systems.
Current high-speed digital transmission systems of the above type include asymmetric, symmetric, high-rate, and very high-rate digital subscriber loops, conventionally known as ADSL, SDSL, HDSL and VDSL, respectively. Normally these and other similar protocols are known as xDSL protocols.
Of these flavors of xDSL, ADSL is intended to co-exist with traditional voice services by using different frequency spectra on the loop. In the future, it is possible that multiple different transmission schemes may be employed in different frequency bands on the same loop, and that these transmission schemes may include traditional analog voice services as well as current and new forms of xDSL. In today's ADSL systems, the plain old telephone services (POTS) use the frequency spectrum between 0 and 4 kHz and the ADSL uses the frequency spectrum between 30 kHz and 1.1 MHz for data over the telephone line.
FIG. 1 illustrates an independent voice circuit and ADSL line interface represented generally by the numeral 10. As is shown in the diagram, the data and voice transmissions use different Digital-to-Analog Converters (DACs) 12. Similarly, the data and voice receptions use different Analog-to-Digital Converters (ADCs) 14.
There is a trend in electronics to manufacture more integrated components. The reasons for this trend include both reducing the cost and reducing complexity of the component. Therefore, it would be beneficial to integrate the circuit as shown in FIG. 1 such that it uses only one DAC 12 and one ADC 14. While this concept may seem trivial, it is complicated by the fact that the timing for each DAC 12 is derived from a separate clock. The situation is the same for each ADC 14. Therefore, the timing of an integrated DAC or ADC will require significant changes to the current technology.
In addition, any of the xDSL systems may be used to transport digitized voice as part of its payload. When a clock domain of the digitized voice and a clock domain of the xDSL bit streams are not synchronous, it can lead to inefficiencies in the framing rate of the voice channels in the xDSL data streams. The asynchronous nature of the clocks can also lead to difficulties with voice sampling clock generation at the customer premises end of the xDSL loop.
It is an object of the present invention to obviate or mitigate some of the above disadvantages.